The present invention relates to a distributed feedback semiconductor laser having a diffraction grating and a current injection method therefor.
In an ordinary semiconductor laser, light intensity in its active layer during oscillation is distributed nonuniformly in the direction of travel of light (hereinafter referred to as the "axial direction"). The nonuniformity of light intensity in the axial direction is particularly remarkable in a phase shifted distributed feedback semiconductor laser (hereinafter referred to as the "phase shifted DFB laser") which has incorporated therein a phase shifted diffraction grating of excellent wavelength selectively. It has been reported that, in the case of such a nonuniform distribution of light intensity, if the injected current density distribution is uniform, the refractive index varies owing to spatial hole burning in the axial direction, by which a threshold gain difference between a main mode and a sub-mode (hereinafter referred to as the "oscillation threshold value gain difference"), which is an index of a single-wavelength operation, varies with an increase in the amount of current injected (see Soda et al., IEEE J. Quantum Electron., Vol. QE-23, pp 804-814, 1987.) That is to say, the higher the light intensity, the more carriers are consumed by stimulated emission, and consequently, the carrier density in that region decreases relative to the light intensity, with the result that the carrier density has, in the axial direction, a nonuniform distribution reverse from the light intensity distribution. On the other hand, the refractive index of semiconductor varies with the internal carrier density and decreases (or increases) when the carrier density becomes high (or low). Therefore the refractive index also has a distribution depending on the nonuniform carrier density distribution. It has been observed not only that this refractive index variation decreases the threshold gain difference, which leads to degradation of the single-wavelength operation, but also that the carrier density variation makes the laser output light readily saturable during a high output operation of the laser.
The present inventors have filed a patent application on a light semiconductor device of a structure in which its electric resistance ratio is distributed nonuniformly and the injected current density is distributed substantially in proportion to the light intensity distribution in the axial direction so as to avoid the above-mentioned problems (Japan. patent application No. 168314/87). This is intended to obtain substantially uniform net carrier density distribution and refractive index distribution by providing an injected current density distribution substantially opposite in direction from the carrier density distribution made nonuniform by spatial hole burning in the axial direction. With such a structure, it is possible to decrease the refractive index variation by spatial hole burning in the axial direction which increases with the current being injected. However, no matter what electric resistance ratio distribution (or injection current density distribution) may be used, a change in the injected current inevitably causes some variations in the refractive index in the axial direction, and consequently, the above-mentioned structure cannot be applied to the case where the dynamic range of current is selected so large.
According to the above method, the injected current density distribution is fixed independently of the current being injected, and consequently, the distribution remains unchanged even during the injection of current below the threshold current. On the other hand, the refractive index variation by hole burning does not occur until oscillation; so that when the injected current is smaller than the threshold current, the refractive index will be changed by the nonuniform injected current density distribution alone. Because of this refractive index variation, the injection of current of an arbitrary value greater than the threshold current does not provide an injected current density distribution which always completely cancels the refractive index variation by hole burning.
As described above, the conventional semiconductor laser device utilizing nonuniform current injection and the current injection method therefor cannot completely suppress the refractive index variation by spatial hole burning in the axial direction, and hence incur deterioration of the single-wavelength selectively and liability to saturation of the laser output light at the time of high current injection, leading to the problem that no stable laser output can be obtained.